Television transmitting apparatus



y 1941- A. D. BLUIMLEIN ETAL 79 TELEVISION TRANSMITTING APPARATUS FiledNOV. 9, 1937 2 Sheets-Sheet-2 l lAlVE/VTORS 414m 0. Bil/MAFIA CECIL 0. anwp A TTORNEY Patented May 27, 1941 TELEVISION TRANSIVHTTING APPARATUSAlan Dower Blumlein, Ealing, London, and Cecil Oswald Browne, WestActon, London, England, assignors to Electric & Musical IndustriesLimited, Hayes, Middlesex, England, a company of Great BritainApplication November 9, 1937, Serial No. 173,550 In Great BritainNovember 10, 1936 14 Claims. (01. 178-72) This invention relates tosignal transmitting systems and more especially to televisiontransmitting apparatus, and is directed to the reduction or eliminationof spurious or undesired signals of irregular wave form and ofrelatively low frequency.

The invention is particularly, although not exclusively, applicable totransmitting apparatus embodying a cathode ray tube picture scanner inwhich a mosaic or other screen structure is provided composed of aplurality of photo-sensitive mutually-insulated elements capacitativelyassociated with a signal plate, the elements acquiring an electrostaticcharge according to the intensity of the corresponding elementary areasof the picture to be transmitted. The elements are restored to anequilibrium potential under the action of a scanning beam of cathoderays, restoration of the elements to the equilibrium potentialgenerating picture signals which are fed to a suitable amplifier fortransmission.

In such transmission apparatus the spurious or undesired signals giverise to what has become known as tilt, and the tilting effect ismanifested in the picture signals by the signals having superimposedupon them a saw-tooth wave so that the picture signals generated whenscanning a line of constant brightness are such as to correspond to aline having a progressively changing brightness along its length. In apicture, reconstituted by means of a cathode ray tube, the image on thefluorescent screen appears to have a progressively increasing brightnesstowards one corner of the reconstituted image.

In order to reduce the effect of tilt, it has been suggested to apply anequal and opposite saw-tooth wave to counteract the saw-tooth wave uponwhich the picture signals are imposed. Such proposal, whilst resultingin a reduction of tilt, is not altogether satisfactory and, accordingly,it is the chief object of the present invention to provide an improvedmanner of reducing the effect of undesired signals.

According to the invention, a method of removing spurious or undesiredsignals of relatively low frequency from signals extending over arelatively wide range including said low frequency undesired signals isprovided which comprises interspersing said signals with other signalsof a datum value and recurrent at a frequency higher than the frequencyof the undesired signals, attenuating frequencies below a predeterminedfrequency so as to remove said undesired signals from the signal chain,and subsequently components of the desired signals by reference to therecurrent signals of datum value. In the case of a televisiontransmitting apparatus of the above described kind to which theinvention is particularly applicable, the picture signals generated byscanning the mosaic or other screen are interspersed with signals ofdatum Value, preferably corresponding to black, of a frequency higherthan the frequency of the tilt component,

v.low frequency signals up to a frequency or higher than or equal tothat of the tilt component being attenuated so as to remove such signalsincluding the tilt component from the picture signals, the suppressedlow frequency or D. C. components of the picture signals beingsubsequently reinserted by reference to the recurrent signals of datumvalue. The picture signals from which .ing to the invention may beproduced by projecting the image of the object of which a picture is tobe transmitted on to the mosaic or'other screen through a line gridwhich casts a shadow on said screen forming black lines on the screenpreferable at right angles to the direction of the line scan wherebywhen said screen is scanned the desired picture signals so produced areinterspersed with signals of datum level, i. .e. signals correspondingto, the black lines formed .by said grid.

The grid may be formed on the support carrying the mosaic or arranged atany other suitable point with reference to the image producing opticalsystem.

The effect of the grid in the receiver may be minimised or renderedrelatively innocuous by moving or reciprocating the grid or by suitabletreatment of the signals during transmission.

In order that the invention may be clearly understood and readilycarried into effect, the same will now be more fully described withreference to the accompanying drawings in which:

Figure 1 is a diagrammatic illustration of a picture scanning apparatusemployed in the preferred form of the invention, and

Figures 2, 3 and 4 are explanatory diagrams.

Figures 5, 6a, 6b, 6c, 6d and 6e are further explanatory diagrams,

Figure 7 illustrates another form of scanning re-inserting suppressedlow frequency or D. C.. apparatus employed in the invention,

Figure 8 is a circuit diagram of a convenient arrangement fortransmitting signals produced according to the invention, while Figure 9shows in block diagram form a wave shaping and combining circuit.

In the following description of the invention with reference to Figuresl-7, the invention will be described in its application to the reductionor elimination of tilt and, by way of example, to a television picturescanning device of the kind in which an image of the object fortransmission is projected onto one side of a mosaic screen, the screenbeing scanned from the same side.

As shown in Figure 1 the picture scanning device comprises an evacuatedenvelope 4 in which is mounted a mosaic screen 5 composed of amultiplicity of photo-electric mutually-insulated elementscapacitatively associated with a signal plate 5. The envelope 4 alsocontain a cathode ray gun I for generating a beam of electrons, the beambeing focussed in known manner and deflected by electrostatic orelectromagnetic means, not shown. An image of the object fortransmission is projected onto the mosaic screen through a lens systemindicated at 8, and on scanning the image by the beam picture signalsindicated by the wave form 9 (Fig. 2) are established across aresistance 60.. The picture signals should be electrical variationshaving a zero or horizontal axis as at l l in Figure 2, but, owing tothe effect of tilt, the electrical variations occur relatively to aninclined axis I2 so that in effect the picture signals represented bythe wave form 9 are superimposed upon a saw tooth wave. These saw-toothoscillations, which are recurrent at about the line frequency will,unless special steps are taken, result in an uneven shadingof thereconstituted picture as stated above.

In accordance with one embodiment ofthe present invention there isinterposed between the lens 8 and the light source, or at any othersuitable position, a grid 10 which has the effect of dividing the imageprojected onto the screen 5 into a plurality of vertical stripsseparated by black lines disposed at right angles to the direction ofscanning. This grid I is placed close enough to the object or virtualobject imaged by the lens 8 to cause definite black and white lines toappear in the image produced on the mosaic screen 5. For example, whentransmitting from film, the grid 8 may be inserted close to the film andlikewise for direct vision the grid 8 may be so placed in front of theobject that it is also in focus. In an alternative arrangement a doublelens system may be used, one lens of the system forming an image of theoriginal object to be transmitted. This image is formed on the gridclose to which is placed a condensing lens which re-condenses the lightinto another lens such as lens 8, which throws an image of the firstimage, which constitutes a virtual object, and of the grid 10 on themosaic screen 5. This alternative, of course, involves an additionalinversion of the image and loss of light and it may be preferable incarrying out the invention to utilise a grid actually applied to thecarrier, as described more fully below with reference to Fig. 7 of theaccompanying drawings. The width of the black lines formed on the screenshould be such that the lines will be resolved during the scanningaction. On scanning a horizontal line of the image the picture signalsgenerated will be interspersed with black signals of datum value asshown in Figure 3.

The signals derived from the scanning action are fed to an amplifierwhich is arranged to have such a low frequency cut-off as by the use ofa resistance-capacity combination in the amplifier, that frequencies ofthe order of the line frequency, that is to say, of the order of thetilt component, are not transmitted. For example, if the line frequencyis 10,000 cycles er second, then it should be arranged that theamplifier has a cut-off at a small multiple of the line frequency as,for example 40,000 cycles.

' Attenuation of the lower frequencies imparts to the signals appearingat the output of the amplifier a wave form similar to thatdiagrammatically shown in Figure 4. It will be seen from this figurethat the low frequency tilt component has been removed and since certainlow frequency components of the picture signals have also been removed,the black signals extend below the electrical axis in the negative senseby an amount dependent upon the amplitude of the attenuated lowfrequencies.

At the transmitter or receiver it is necessary correctly to relate thepicture signals with reference to their true zero axis, that is to say,to re-align the negative peaks of the black signals relatively to ahorizontal axis. This is effected by reinserting the suppressed picturefrequencies at a suitable point in the transmission channel, byreference to the recurrent black signals in a manner similar to thatreferred to in the specification of British Patent No. 422,906. For theproper functioning of the apparatus, the time constant of theresistance-condenser combination of the reinserting device is longcompared with the interval between the successive black signals butshort compared with the highest of the suppressed frequencies. Forexample, if the number of black lines produced by the said grid is andthe line frequency is 10,000 cycles per second, the frequency of therecurrent black lines will be one megacycle, and the interval betweensuccessive black signals will be one micro-second. If the time constantof the reinserting device is of the order of 1-10 micro-seconds, it willbe seen that the time constant is long compared with the intervalsbetween successive black signals, i. e. one micro-second, but shortcompared with the period of 25 micro-seconds of the lowest frequency of40,000 cycles effectively transmitted.

In the case of a 400 line picture the grid [0 would probably havesomewhere in the neighbourhood of 400 bars or lines, the limit to thenumber of bars being set by the resolving power of the scanning system.The grid 50 is shown in Fig. 1 as comprising a small number of bars onlyfor convenience in drawing.

It will be appreciated that the resistance-capacity combination which isusually associated with a diode for reinserting purposes, may, in somecases, be utilised to afford the required cut of the lower frequencies.

It will be understood that, providing the recurrent black signals arecaused to appear in the output of the picture scanner, the tiltcomponent can be removed either at the transmitter in which case thesuppressed frequencies can be reinserted at a suitable point at thetransmitter or receiver, or the frequencies may be both suppressed andreinserted at the receiver.

"tilt effect is eliminated or substantially reduced. I

If desired, steady light, that is to say, a uniform auxiliaryillumination apart from the projected image, may be projected onto themosaic screen through the same grid I to enable signal of the gridfrequency to be obtained even when the picture is dark. This enables theD. C. reinserting device to work more satisfactorily for the dark partsof the picture.

It is possible that the black lines produced on the mosaic screen mayhave some noticeable effeet in the reconstituted picture and for thepurpose of rendering such lines less noticeable it may be arranged thatthe position of the grid relatively to the mosaic is continuouslychanged during the scanning action. Thus, by the pro vision of suitablemechanism and by mounting the grid in a suitable manner, the grid may beintermittently or continuously reciprocated so that the position of the:black lines cast upon the screen are changed or interlaced duringappropriate periods. Suitable means for reciprocating the grid may be aneccentric IUI and I03 shown in Fig. 1 after the fashion shown by thescanning arrangement in Fig. 3 of page 18 of the textbook entitled DasElektrische Fernsehen und das Telehor by Mihaly, published in Berlin byM. Krayn in 1926.

It will usually be advisable to provide a filter to eliminate theelectrical frequency resulting from the scanning of the black lines,such frequency in the case considered above being 1 megacycle.

An alternative method of removing the efiects of the black lines on thereceived picture is to use in the transmitter an arrangement in whichthe width of the black lines is equal to the width of the spaces betweenthem, and utilise prior to transmission or at the receiver two sets ofpicture currents in which the D. C. components have been re-inserted,one set being derived from the other and delayed by an amountcorresponding to the width of a black line. The two currents addedtogether produce an effect as shown in Figure 5 of the drawings in whichthe D. C. component has been added and in which the full line arepresents the wave form of the undelayed current and the dotted line brepresents the wave form of the delayed current. It will be seen that inthe wave form produced by adding the two currents, the valleyscorresponding to black in the original current are largely filled up,though as the component currents do not entirely match, the valleys areby no means eliminated.

A more nearly complete-elimination of the valleys can be obtained byusing the method illustrated in Figures 6a to 6e of the accompanyingdrawings. Figure 6a shows a typical wave form for a picture line withoutany interruptions due to interspersed black signals. Figure 6b shows thewave form of a current of a quarter the amplitude of that shown inFigure 6a with interruptions due to interspersed black signals producedfrom black lines of the same width as the spaces between them. Figure 6cshows the wave form of a current similar to that of Figure 6b but halfthe amplitude of the desired wave shown in Figure 6a and delayed by anamount equivalent to the width of a black signal, and Figure 6d showsthe wave form of a current similar to that of Figure 6b except that ithas been delayed by an amount equivalent to the width of two blacksignals. If the currents with the wave forms of Figure 1.

shown in Figures 6b, 6c,and 6d are added together, a current with a waveform as shown in Figure 6e will be obtained. The wave form in Figure 66will be seen to correspond very closely with that of Figure 6a., andwill thus give a picture from which the effect of the black bars hasbeen practically completely eliminated. The delaying and combining ofthe impulses may be effected after the fashion and the method disclosedin Patent No. 2,132,655 which issued to J. P. Smith for System forproducing electrical impulses on October 11, 1938, which was filed inFebruary 28, 1935. Alternatively, an arrangement of attenuators andphase delay networks may be used, as described in the Wiener et al.Patent No. 2,024,900 entitled Electrical network system.. Such analternative circuit is shown in Fig. 9 where the input is fed inparallel to a series of attenuators I5I, I53 and I55 for providing theproper amplitudes of the currents to be combined, while the phase delaynetwork I57 provides a delay equal to the width of a black signal andthe phase delay network I55 provides a delay equal to two black signals.

Whilst picture signals in the specific example described areinterspersed with black signals through the interposition of a gridbetween the light source and the lens system, other means, may, however,be employed for the same purpose, as for example, by using a transparentphoto cathode with opaque lines provided on its non-sensitive sidethrough which the image is projected.

By way of illustration, a modified form of picture scanning apparatusaccording to the invention is illustrated in Figure 7. In thisarrangement the tube 4' is arranged with the mosaic screenIb'perpendicular to the axis of the neck in which the cathode ray gun Iis housed, the image projecting optical system 8 being arranged on thisaxis but on the opposite side of the screen I6 from the gun I.

It will be seen that in the arrangement of Figure 7, the grid Iii infront of the lens system 8 is omitted and a screen I6 shown inperspective is provided, to one side of which a grid having lines 20 isapplied. In this arrangement the screen I6 is transparent and formed,for example, of a sheet of mica on one side of which is provided with amultiplicity of mutually insulated photo-sensitive elements constitutingthe mosaic 5 whilst the other side, facing the lens system 8, the sheetis provided with the lines 20 which correspond in function to the gridI0 These lines may be formed by depositing a suitable metal such assilver on the surface of the mica, as for example, by evaporating silverthereon through a suitably shaped grid. The grid is removed afterdeposition of the silver. Alternatively, in some cases, the requirednumber of lines may be preformed as a grid and secured in any suitablemanner to the mica sheet. In either case the lines will be formed ofconducting material and so arranged as to constitute the necessaryconductive signal plate from which the picture signals are obtained onscanning the screen. The image is projected on to the surface of thescreen I6 carrying the lines 20 and the screen scanned in the usual waywithout the necessity for any keystone correction arising, the provisionof the lines 20 causing the photo-sensitive elements in front of thelines to be maintained at an equilibrium potential so that on scanningthe screen, those elements 0pposite the. lines cause excursions in theblack direction in the required manner.

The arrangement according to the invention also enables an accuraterepresentation of average brightness to be obtained, as the directcurrent component is reinserted. If (as for example, clue to the use ofa large number of lines in the grid It) the scanning device cannotresolve the lines efficiently the excursions towards black may be of toosmall an amplitude, so that on reinserticn of the low frequencies, theseare reinserted at a false and too low amplitude, then a correction maybe made after reinsertion, whereby the low frequencies are relativelyboosted to their correct relative amplitude.

Referring again to Figure 4 of the drawings it will be seen that thewave form of the picture signals generated on scanning the screen of thetransmitting tube after frequencies of the order of the tilt componenthave been removed, is composed of picture signals interspersed withblack signals recurrent at a frequency depending on the number of stripsinto which the image is divided and the frequency of the scanning beam,the amplitude of the black signals extending above and below the zeroaxis owing to the fact that certain low frequencieshave been removed.Now this wave form may be employed in a sense as a carrier frequency forthe transmission of the picture signals through the transmittingapparatus, the frequency of the carrier wave form being the frequency ofthe black intervals. In some cases it may be desirable to divide theimage into a number of strips the width of each strip being equal to thespaces between the strips to produce a wave form as shown for example bycurve a. of Figure 5. With such an arrangement the use of the blackintervals as a carrier is particularly advantageous. The carrier wave inthis case is of substantially rectangular form, the peaks of the Waveform being in eifect modulated by the true picture signals. Thegenerated signals may be amplified by an arrangement such as that shownin Figure 8 of the drawings comprising low-pass or bandpass couple'damplifying stages including valves 31 and 32, shown as being of thescreened-grid pent-ode type, the mid-frequency of the bands beingcentred at the carrier frequency. The

amplified signals are then rectified, the rectifier signals so obtainedcontaining the required picture frequencies together with their D. C.component but being substantially free from frequencies of the order ofthe tilt component. For the rectification of the signals it is preferredto employ a full wave rectifier such as shown at 33 in Figure 8, wherebyboth positive and negative portions of the carrier are rectified so thatthere appears in the output of the rectifier no signals of the frequencygenerated by'scanning the black lines at the transmitting tube. If theblack lines in the wave form generated are represented by excursions inthe negative direction, those negative excursions are converted intopositive signals by the full wave rectifier. They therefore appear ofthe same brightness as the white signals immediately before them, andhence will be less noticeable in a picture reproduced from signalstransmitted in accordance with the invention.

The arrangements of the circuits of valves 3| and 32 and of therectifier 33 of Fig. 8 form no part of the present invention and may beof any suitable form known to those skilled in the art. The interruptedor chopped picture signals produced according to the invention may beapplied to the input terminals shown at 34 and the output or thearrangement taken from output terminals such as 35. Valves 3| and 32 inthe arrangement shown are arranged in a low pass filter amplifier,having shunt input capacities constituted by the capacity between thecontrol grid and cathode of valve 3|, the anode circuit of valve 3|including an inductance 36 and resistance 31 connected to a source ofpotential indicated by the sign to the right hand side of the drawingand thence to earth. Inductance 35 is tapped down and connected bycoupling condenser 38- to the control grid of valve 32, the anode ofvalve 32 being connected to the said source of positive potentialthrough the primary of an output transformer 39, which is shunted byresistance 43. The grid leak resistances M and 42 are provided forvalves 3! and 32 respectively, together with grid biasslng resistances43 and 44" arranged in the respective cathode leads and shunted by theappropriate by-pass condenser 45 or 46.

Each end of the secondary of transformer 39' is connected to an anode ofrectifier 33 and a midpoint connection is provided from the secondary ofthe transformer to the output of the rectifier which is taken fromacross the combination including resistance 48 and condenser 49 arrangedin parallel. The positive potential for'the. anodes of rectifier 33 maybe applied through a connection to the upper of the output terminal 35.The cathodes of all the valves 3|, 32, and 33 are connected to a commonearth lead 50 as shown.

It will of course be understood that the rectifier output will be fedafter further amplification, if desired, to a modulating stage formodulating a high frequency carrier prior to transmission.

Although the invention in the specific description has been appliedtowards a reduction of the tilt component arising in a cathode raytransmitter tube, it will be understood that the invention is notlimited thereto, since it can be empioyed to eliminate or reduce otherundesired signals of relatively low frequency arising in. any manner.

We claim:

1. The method of removing spurious signals of relatively low frequencyfrom desired signals generated simultaneously with said spurious signalsextending over a relatively wide range including said spurious lowfrequency signals, which comprises the step of combining said spuriousand desired signals with signals having at least one predetermined datumvalue and recurrent at a frequency higher than the fundamental frequencyof the spurious signals, attenuating frequencies below a predeterminedfrequency so as to remove said spurious signals from the combinedsignals, and subsequently reinserting suppressed low frequencycomponents of the desired signals by reference to the recurrent signalsof predetermined datum value.

2. In a picture transmission system in which an image of the object ofwhich a picture is to be transmitted is resolved into picture signals byprojecting the image onto a mosaic having elements which acquire anelectrostatic charge according to the intensity of elemental imageareas, which mosaic is periodically scanned to discharge said elementsto generate picture signals, the method of removing spurious signalshaving low frequency components, which comprises the steps of combiningthe picture signals generated on scanning the mosaic with signals of apredetermined minimum value and of. a. frequency higher than thefundamental frequency of the spurious components, attenuating allfrequencies below a predetermined value, said predetermined value offrequency being higher than the frequency of the spurious signals to beremoved, and subsequently reinserting the attenuated low frequencycomponents of the picture signals in accordance with the value of therecurrent signals of predetermined minimum value.

3. In a picture transmission system in which an image of the object ofwhich a picture is to be transmitted is resolved into picture signals byprojecting the image onto a mosaic having elements which acquire anelectrostatic charge according to the intensity of elemental imageareas, which mosaic is periodically scanned to discharge said elementsto generate picture signals, the method of removing spurious signalshaving low frequency components which comprises the steps of combiningthe picture signals generated on scanning the mosaic with signals of apredetermined minimum value and of a frequency higher than thefundamental frequency of the spurious components, feeding the combinedsignals to a transmitter, attenuating at the transmitter all signalsbelow a predetermined value of frequency, said predetermined value offrequency being higher than the frequency of the spurious signals to beremoved, and thereafter reinserting the attenuated low frequencycomponents of the picture signals.

4. The method claimed in claim 2 in which the combined signals ofpredetermined datum level have a time duration substantially equal tothe time duration of the picture signals between them and comprising thefurther step of rectifying the signals following attenuation to providethe reinserted low frequency component.

5. In a picture transmission system in which an image of the object ofwhich a picture is to be transmitted is resolved into picture signals byprojecting the image onto a mosaic having elements which acquire anelectrostatic charge according to the intensity of elemental imageareas, which mosaic is periodically scanned to discharge said elementsto generate picture signals, the method of removing spurious signalshaving low frequency components, which comprises the steps of combiningthe picture signals generated on scanning the mosaic with signals of apredetermined minimum value and of a frequency higher than thefundamental frequency of the spurious components, the duration of saidcombined signals being substantially equal to the interval betweensuccessive signals, attenuating all frequenciesbelow a predeterminedvalue, said predetermined value being greater than the frequency of thespurious signals to be removed, rectifying the at.- tenuated signals,and reinserting signal energy proportional to the rectified energy andthe attenuated signals.

6. In a picture transmission system in which an image of the object ofwhich a picture is to be transmitted is resolved into picture signals byprojecting the image onto a mosaic having elements which acquire anelectrostatic charge according to the intensity of elemental imageareas, which mosaic is periodically scanned to discharge said elementsto generate picture signals, the method of removing spurious signalshaving low frequency components, which comprises the steps of combiningthe picture signals generated on scanning the mosaic with signals of apredetermined minimum value and of a frequency higher than thefundamental frequency of the spurious components, the duration of saidcombined signals'being substantially equal to the interval betweensuccessive signals, attenuating all frequencies below a predeterminedvalue, said pre- :determined value being greater than the frequency ofthe spurious signals to be removed, full transmitted is resolved intopicture signals by projecting the image onto a mosaic having ele-.-ments which acquire an electrostatic charge according to the intensityof elemental image areas, which mosaic is periodically scanned todischarge said elements to generate picture signals, the method ofremoving spurious signals having low frequency components, whichcomprises the steps of combining the picture signals generated onscanning the mosaic with signals of a predetermined minimum value and ofa frequency higher than the fundamental frequency of the spuriouscomponents, the duration of said combined signals being substantiallyequal to the interval between successive signals, feeding theinterspersed signals to a transmitter, attenuating the combined signalsat the transmitter selectively below a predetermined frequency, saidpredetermined frequency being higher than the frequency of the spurioussignals to be removed, rectifying the attenuated combined signals, andmodulating the transmitter with both the attenuated combined signals andenergy proportional to the rectified signals.

8. The method claimed in claim '7 and comprising in addition the stepsof delaying a series of picture signals by a predetermined amount, andmixing a set of substantially undelayed picture signals with the delayedpicture signals.

9. The method claimed in claim 7, comprising in addition the step ofmixing undelayed picture signals with signals of the same amplitudedelayed by a predetermined amount, said predetermined amount of delaybeing substantially equal to the time duration of a single said combinedsignal.

10. The method claimed in claim 7 comprising in addition the step ofmixing together undelayed picture signals having interspersed signals ofa predetermined minimum value, picture signals of twice the amplitude ofthe undelayed signals and delayed by an amount equal to th time durationof a single interspersed signal of predetermined minimum value, andpicture signals of the same amplitude as the undelayed signals, butdelayed to an extent corresponding in time duration to twice theduration of a single interspersed signal of predetermined minimum value.

11. A picture transmission system comprising in combination a picturesignal generating device including a mosaic having elements whichacquire an electrostatic charge according to the intensity of elementalimage areas, means for periodically scanning said mosaic to dischargesaid elements to generate picture signals, means for projecting an imageto be transmitted onto the mosaic, a grid in association with said imageprojecting means, said grid being adapted to cast a shadow on saidscreen in such manner that when said screen is scanned interspersedsignals of a predetermined level and frequency are produced with saidpicture signals. means for removing signals below a predeterminedfrequency lower than the frequency of said inter-- spersed signals, andmeans for subsequently rcinserting suppressed low frequency componentsof the picture signals by reference to the recurrent signals of datumvalue.

12. A television or picture: transmission system according to claim 11wherein said grid is adapted to be moved reciprocally during thescanning of the mosaic whereby an image of said grid will be lessapparent in the received picture.

13. A television or picture; transmission system according to claim 11in which said mosaic screen is transparent the optical image v0i theobject 135 of which a picture is to be transmitted projected onto theopposite side of the screen from :that on which the mosaic is formed,said grid lacing applied to said opposite side and being formed ofopaque electrically conducting material which forms the conductivesignal plate of the scanner.

14. A television or picture transmission system according to claim 11 inwhich said mosaic screen is translucent the optical image of the obj ofwhich a picture is to be transmitted being projected onto the oppositeside of the screen from thatch which the mosaic is formed, said gridbeing applied to said opposite side and being formed of opaqueelectrically conducting material which forms the conductive signal plateof the scanner.

ALAN DOWER BLUMLEIN. CECIL OSWALD BROWNE.

